The present invention relates to an optical disk for optically writing and reading information and to a method for manufacturing the same.
An optical disk is known in which information is written by a laser beam modulated by the information, and the information is thereafter photoelectrically read by a laser beam.
In this type of optical disk, an optical recording layer of metal or photosensitive dye is covered on a substrate of glass or plastic whose surface is finished as an optical plane. As shown in FIG. 1, information is written on this optical disk 1 by rotating it in the direction shown by the arrow while irradiating an information-modulated laser beam L on the surface of the optical disk 1 with a lens 2 and moving a reading/writing head 3 in the direction of the arrow. This device incorporates a high precision tracking control mechanism for indirectly forming tracks to define the rotation of the optical disk 1 and the travelling path of the reading/writing head 3, since optical disk 1 does not have any guiding tracks. However, such a control mechanism tends to be mechanically complex and costly.
In order to more easily perform tracking for writing, an optical disk 4 has been proposed according to which, as shown in FIG. 2, a guiding groove 6 having a width of 0.6 .mu.m and a depth of 1/8 of the wavelength (.lambda.) of the laser beam L is formed on a substrate 5 at a track pitch of 1.67 .mu.m, and an optical recording layer 7 is coated on the surface of this substrate. Such an optical disk is known, for example, from K. Bulthuis, et al, "Ten billion bits on a disk", IEEE, spectrum, Aug. p.26, 1979. When this grooved optical disk 4 and the planar optical disk 1 described above are compared the laser beam L which is focussed by the lens 2 and irradiated as a spot of diameter "d" on the optical disk 1 is reflected by the surface of the optical disk 1 and returns to the lens 2, as shown in FIG. 3. The power intensity "I" at the surface of the lens 2 of this reflected light generally has single-peaked power intensity distribution of I(.gamma.). This reflected light catches I(.gamma.)NA as defined by the effective aperture NA of the lens 2 and is guided to a photodetector. With the planar optical disk 1, since I(.gamma.) is constant at any position of its surface, information cannot be recorded on the optical disk 1 at a desired track pitch unless the rotation of the optical disk 1 and the feeding of the lens 2 in the radial direction of the disk are correctly programmed in advance for indirect control.
On the contrary, with the grooved optical disk 4 shown in FIG. 2, the phase of the light reflected to the center of the surface of the lens 2 from the bottom surface of the guiding groove 6 having a depth of 1/8 of the wavelength of the laser beam lags that of the light reflected from the periphery of the guiding groove 6 by 1/4 wavelength. Thus, the reflected light rays caught at the center of the lens interfere with each other so that the power intensity is reduced. The total reflected light is diverged by the diffraction by the guiding groove 6 and is widely distributed over the surface of the lens 2. Thus, the power intensity distribution of the reflected light on the surface of the lens 2 becomes that as shown in FIG. 4. When the center of the irradiating spot of the laser beam L is aligned with the center of the guiding groove 6, the distribution becomes a symmetrical reflected light distribution of I'(.gamma.) wherein the power intensity at the center of the lens 2 is low. When the center of the spot is deviated from the guiding groove 6, the distribution becomes the distribution I(.gamma.) with maximum central power intensity. Thus, when variations in the power intensity and the distribution of the reflected light incident in NA of the lens 2 are converted into electrical signals by the photodetector disposed behind the lens 2, tracking may be easily performed so that the spot center of the laser beam L from the reading/writing head 3 is constantly aligned with the center of the guiding groove 6.
This guiding groove 6 is conventionally formed by photoetching methods utilizing a laser beam. However, as has already been described, it is very difficult to form, uniformly and with precision, guiding grooves of fine width and depth. This factor has significantly increased the cost of optical disks.